In this brief review a survey is presented of our present knowledge of mitochondrial and peroxisomal fatty acid oxidation. For mitochondrial fatty acid oxidation, emphasis is placed on the key regulatory role of the carnitine acyltransferase system and its modulation by malonyl-Co-A. Also, the concept of opposite regulation of fatty acid synthesis and oxidation in response to nutritional or hormonal stimuli is presented. For peroxisomal fatty acid oxidation, the key features of the system are summarized together with various approaches used to measure the contribution of the peroxisomal pathway to overall fatty acid oxidation. The conclusion is reached that this contribution is minor, at least for the most abundant fatty acids palmitate and oleate.